This invention relates to color cathode ray tubes and more particularly to the temporary modification of the apertured pattern member utilized in the forming of a patterned color screen structure.
Color cathode ray tubes employed in the presentation of multi-color display imagery, such as color television, usually have patterned screens comprised of repetitive groups of related phosphor materials. These phosphor groupings are of various shapes of which dot-like areas are a common deposition.
An apertured pattern member is usually positioned in spaced relationship with the screen, which in a post deflection type tube, functions as a grid in the finished tube and is usually priorly utilized in disposing the patterned screen on the interior surface of the face panel. In the well-known shadow mask tube construction, the screen pattern is also formed by utilizing a spatially oriented multiple aperture member. Regardless of which tube structure is considered, each of the openings in the apertured pattern member is related to specific grouping of phosphor elements in a spaced manner to enable selected electron beams traversing the apertures to impinge the proper pattern elements therebeneath. Usually the individual phosphor elements of the screen pattern are separated from one another by relatively small interstitial spacings which enhance color purity by reducing the possibility of adjacent color-emitting elements being excited by a specific electron beam.
It has been found that improved contrast of the color screen image can be achieved by disposing an opaque light-absorbing material in the interstitial spacing between the respective phosphor elements. In essence, each of the phosphor element is then surrounded or defined by a substantially dark encompassment which collectively comprise a multi-opening pattern disposed in the panel in the form of a windowed webbing having an array of substantially opaque connected interstices. While such web-like screen structures have been fabricated, either before or after phosphor screening, it has proven to be expeditious to form the windowed webbing prior to the deposition of the phosphor elements of the screen. Such web-like structures have been fabricated by several known processes wherein photo-deposition techniques play a prominent part. An example of a typical web-forming process is disclosed in Ser. No. 41,535 by R. L. Bergamo et al., filed May 28, 1970, and assigned to the assignee of this invention.
To heighten the contrast and improve registration, it has been found beneficial to have at least some of the openings in the spatially associated apertured pattern member to be of a size equal to or larger than the window areas in the opaque webbing. This aperture-to-window relationship is referenced in the art as "negative guardband" or a "window-limited" screen. In this type of screen construction, when a phosphor dot is impinged by an aperture-sized beam, the excited phosphor area completely fills the associated window area with a luminescent hue.
Several techniques have been employed to achieve a multiplex window-limited color CRT screen structure in which the window openings in the opaque interstitial webbing are smaller than the associated apertures in, for example, a shadow mask apertured member subsequently utilized in the finished operable tube.
In accordance with one screen forming procedure, wherein the sizes of the basic mask apertures remain fixed, a pattern of clear polymerized polyvinyl alcohol dots is light disposed on the interior of the panel, on those areas subsequently to be windows in the opaque webbing, by photo exposure through the related apertured shadow mask. After development, the resultant island-like polymerized dots are reduced in size by an erosion technique involving a chemical degrading agent. Next, an opaque graphite coating is applied to completely overcoat the pattern of the reduced-in-size clear dots and the adjacent bare interstitial glass areas. Then, an oxidizing agent is applied to completely degrade the pattern of dots thereby loosening the superjacent opaque coating thereon, whereupon the materials so loosened are removed by a subsequent water development step. Thus, there is formed an opaque interstitial web having multitudinous windows defined as bare glass areas that are of a size smaller than the related mask apertures. The phosphor pattern elements are then disposed on these window areas upon photo-exposure through the same size mask apertures by one of the various processes known to the art. While the aforedescribed dot-erosion procedure is an acceptable production technique, it necessitates the inclusion of additional process steps.
By another procedure, after the dot-initiated windows and the overlaying phosphor elements are formed by a separate series of photo exposures through the initially apertured mask, the mask apertures are subjected to a chemical etching process to enlarge their sizes thereby effecting the desired dimensional differential between the final-sized apertures and the formed windows in the interstitial webbing. While, this too, is a production procedure, the aperture etching requires additional closely controlled processing steps. In addition, as a result of this aperture etching procedure, metallurgical inconsistencies of the mask material have been evidenced such as a ragged aperture periphery, a weakening of the mask material per se, and destruction of the desirable dark oxide coating on the surface of the mask. Furthermore, with reference to the economics of tube production, etching of the mask apertures is an inherently costly procedure as it precludes any subsequent reuse of masks which ordinarily could be salvageable from the final stages of the tube manufacturing operation.
The prior art is replete with a variety of techniques for modifying the sizes of the shadow mask apertures for utilization in the forming or operation of specific types of color screen structures. In several disclosures the changing of aperture sizes is executed by the deposition within the aperture openings of peripheral fill-in substances applied, as for example, by painting, dipping, electrophoresis, electroplating, and vaporization.